The radical species derived from cyclopentane, cyclopentenes and cyclohexenes by Co-60 gamma-ray irradiation at 77 K in halocarbon matrices have been studied by ESR spectroscopy. The electronic and geometrical structures of the radical species are discsssed on the basis of the experimental hyperfine coupling constants and semiempirical MO calculations. Exposure of the tetramethylsilane frozen solution containing cyclopentene to Co-60 gamma-rays produced specifically 3-cyclopenten-1-yl.